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carrier GB 30 175

Hello need help my customer has a gb 30175 650 it is set up for glycol it get code 68 and 69 in a low load situation have replace the sensors on the comp and the barrel also have change out the control pc board it will make temperature all day long but when it gets down to temp it gets the codes.
thanks for any help i can get
Allen l svc

Have you also checked the cooler saturation temperature sensor and verified that it is giving the correct resistance. This is located at the cooler barrel inlet/liquid refrigerant just after the EXV. If this sensor is not working correctly it could also give you this error code as the EXV will not be feeding refrigerant correctly.

It is possible that the performance of the sensor has degraded and is now resulting in an incorrect voltage drop. You should be able to test by turning off/stop the chiller and let sit idle for 20 minutes or so and compare the voltage drop at the board. Its possible to use suction pressure to determine temperature but consider that it could be/will be in pump down.

As an example @ 77 degrees the voltage drop will be 2.147 VDC and Ohms resistance value of 4976